1. Field of the Invention
The present invention relates to a method and a sensor for determining the local contrast of an observed scene by detecting the luminance of said scene using an array of CMOS photosensors.
2. Description of the Related Art
In the optical imaging art, it is known to acquire the image of an observed scene using an array of photosensors each of which is associated with an analysis circuit forming with it what is usually referred to as a pixel. The array of pixels is preferably implemented in the form of a CMOS integrated circuit.
The photosensors of each pixel deliver a current proportional to the quantity of light that they receive from the observed scene. In practice, the average brightness of a real image may vary as a function of situation by six orders of magnitude. Consequently, the currents delivered by the photosensors may vary in the same proportions. It is therefore necessary to use adaptation circuits to adapt the currents to the levels required by the processing circuits of each pixel, which is to the detriment of the input dynamic of the system, which in practice rarely exceeds two to three orders of magnitude.
Several techniques for increasing the input dynamic are known in the art. One of them, described in a paper by S. Kavidias et al. in IEEE, Journal of Solid State Circuits, vol. 35, August 2000 entitled “A Logarithmic Response CMOS Image Sensor with On-Chip Calibration”, consists in applying logarithmic compression to the current delivered by the photosensor, using low-inversion MOS transistors. The problem with this technique is that it requires matched transistors to effect logarithmic compression within the matrix of pixels. This is a considerable implementation constraint if anything like satisfactory accuracy is required.
Another technique, described for example in FR 2 605 475, in WO 98/14002 and in a paper by Y. Ni et al. in IEEE, Journal of Solid State Circuits, vol. 32, July 1997 entitled “Histogramme-Equalization-Based Adaptive Image Sensor for Real-Time Vision”, consists in integrating the current delivered by the photosensor in a capacitor until a defined reference voltage is obtained across the capacitor. Each pixel of the array of pixels signals the time at which the voltage across its integrator capacitor reaches the reference value. In this case, the input dynamic is limited by the maximum integration time allowed to the system and by its intrinsic noise. This technique may be associated with methods of classifying samples using histograms. This technique is also complex and leaves something to be desired with regard to accuracy.
To overcome the problems caused by the considerable variation in brightness, it is also known in the art to determine the local contrast of an image captured by an array of pixels. This method is of interest because, for a given element of the observed scene, the contrast has the same value whether that element is in the shade or in bright sunlight. Now, as the local contrast provides a reliable representation of the activity of each pixel, determining it eliminates the problems inherent to the prior art techniques described in the prior art documents cited above.